Size reduction of a spark plug is desired for purposes such as increasing the degree of freedom in designing an internal combustion engine. For example, a spark plug with a nominal diameter of the thread of the metal shell of not more than 10 mm has been developed. On the other hand, there are increasing tendencies to desire the airtight and dielectric strength properties of spark plug due to an increase in the compression of fuel gas in internal combustion engines, and an accompanying increase in the voltage applied to the spark plug.